1. Field of the Invention
The present invention relates to a device for determining the color value of a light flux.
2. Description of Related Art
In general, in the determination of the color value of a radiation, three partial light fluxes of the radiation to be measured are generated and guided via three color filters to photodiodes or phototransistors and converted by the latter into three corresponding electrical signals. On the other hand, the whole light flux to be measured can also be decomposed by an optical grating or a prism into its spectral components, and these are projected onto a diode array, which most often consists of 256 or 512 diodes.
In the case of the decomposition of the whole light flux into three partial light fluxes, this is referred to as an RGB measurement of the light, since this measurement is most often undertaken in the three color ranges red, green and blue. Each of the three color filters respectively allows a different part of the wavelength range of the light flux picked up to pass to the photodiode connected downstream of it. The photodiodes emit a current, which is proportional to the intensity of the loading and to its own sensitivity, used as a measuring signal. The color information obtained here is most often sparse, since the spectral colors lying between the three basic colors red, green and blue are determined only in a manner reduced by the respective filter factor. Also, the color filters normally used for an RGB measurement have a broad pass band of about 100 nm and, accordingly, an average value of the intensity over the entire pass band is measured by the diode connected downstream.
In general, it can be established that the further color components lie from each other on the color circle or in another adequate colors representation, then the closer their mixed value lies to the colors white or grey. If colors are measured only in terms of the components red, green and blue, only these values are available for a later synthesis of these colors and the colors composed from these values then appear milky white in the case of light shades or dull to grey in the case of dark shades. Brilliant colors are, in contrast, always pure spectral colors or mixtures of closely adjacent spectral colors.
A color measurement leading to brilliant colors is achieved with the aid of the spectral decomposition of the light flux to be measured, by means of an optical grating or a prism, as already mentioned above. Unfortunately, considerable light losses result in both arrangements. In the case of the grating, losses result from the fact that the other orders of diffraction also receive light. In the case of prisms, in particular for short overall lengths of the optical equipment, a multiple prism arrangement is necessary, which exhibits strong reflection and absorption losses and by means of which the enclosed space is in stark contradiction to the often necessary miniaturization of the measuring electronics. Nevertheless, the spectrum generated is mostly only of a short length and diode arrays are therefore almost exclusively used, of which the most conspicuous disadvantage is that they can read out only slowly. Also, the fine spectral resolution requires a correspondingly narrow and thus unfortunately low-light gap, since otherwise the images of the gap in the individual wavelength ranges overlap and destroy the resolution achieved by means of the grating or prism. This effect slows down the reading-out considerably.
A device of the generic type for determining the color value of a light flux is disclosed in a brochure of the firm OCLI, 2789 Northpoint Parkway, Santa Rosa, Calif. 95407-7397. This known device has a linear graded interference filter, which has only one expansion direction and through which a light flux is incident on a number of photodiodes, which are arranged adjacent to one another in the expansion direction of the graded interference filter and are used to supply measuring signals, on the basis of which the color value is calculated. The photodiodes are present in a 256-diode array, which is connected to a corresponding evaluation circuit for the measuring signals. Focusing optics are arranged in front of the graded interference filter, in order to focus the light flux coming from a halogen lamp, after reflection at a sample, onto the photodiodes.